The present invention relates to electric discharge devices and, in particular, to a non-conductive collar for mounting in a conductive shell in a non-conductive tube.
Various lighting devices such as neon lights employ an electrode that is mounted within a hermetically sealed tube containing a rare gas. A known electrode includes a cylindrical metallic shell mounted on leads that support the shell coaxially in a glass tube. The metallic shell is internally coated with an electron emissive material.
In the manufacture of quality electrodes, a ceramic collar (made for example from STEATITE L4 or L5) is inserted into the open end of the metal shell that acts as an electron emitting source. The collar is attached during a shell rolling step where the shell is crimped into an annular dovetail recess on the collar.
The purpose of the collar during normal lamp use is to collimate the arc stream generated inside the shell and to restrict the flow of electrons that tend to blacken the fringe areas of the phosphor coated neon tube. By collimating the electron beam, the collar increases the efficiency of the operation. The collar also prevents the sputtering that might otherwise occur on the conductive shell. The sputtering erodes the electrode as well as blackening the inside of the glass tube in which the tube is mounted. This blackening is aesthetically undesirable and can degrade performance. The flange of the collar can also prevent electrons flowing through the collar from returning to the outside of the shell.
The collar acts to collimate the arc stream and gives rise to uniform heating of the shell during a bombardment cycle when large currents are applied during lamp manufacture. One conspicuous problem with the collar, however, is the arcing and blackening that takes place during said bombardment. A tight seal between the shell and collar can reduce or eliminate this undesirable effect, but the tightness of the seal between the shell and collar is limited by the compressive strength of the ceramic.
FIG. 3 shows a technique according to the prior art for sealing a shell 14 and collar 42. The dovetail locking area denoted by 44 is considered conventional with the lip of the shell 14 remaining open at about its original diameter. In these prior designs the force used during the shell sealing operation is limited by the compressive strength of the collar. Cracking can occur when attempting to achieve a good metal to ceramic seal.
In U.S. Pat. No. 1,984,482 the metallic shell of the electrode is capped by a ceramic disc or cap. The device is held either by studs on the shell or by thermally crimping the glass tube containing the electrode. This reference does not show, however, a technique for achieving a tight shell to collar seal.
Russian references 1,472,972 and 1,026,193 both show cylindrical electrode shells holding at their outer ends an insulating bushing. These bushings have a hollow cylindrical neck and a flange of a greater diameter. The neck flares in a direction away from the flange. The flaring leaves a valley with a low point at the junction between the flange and the neck. The cylindrical electrode shell is crimped over the neck and into the valley. A disadvantage with this type of crimping is that the crimping forces are applied at the valley at a point where the material thickness is minimal. This crimping action either restricts the crimping force or tends to crack the bushing.
See also U.S. Pat. Nos. 1,949,276; 2,271,658; 3,369,143; 3,636,401; 4,065,691; 4,092,560; 4,611,145; and 4,825,126.
It is an object of this invention to allow the neck sealing operation to proceed with the force necessary to seal tightly these components. Thus, there is a need for an improved insulating collar that can be tightly crimp-mounted in an electrode shell without cracking.